Method and apparatus for automatic high power tube recovery

ABSTRACT

The invention consists, generally, of a method for automatically reducing power to an electron emitting cathode by sensing an operating condition of the electron emitting cathode, calculating a condition number based upon the operating condition, comparing the condition number to a threshold value, and reducing the power to the electron emitting cathode when the condition number is greater than the threshold. The apparatus and method may be implemented upon a system having a high voltage power source, an RF tube, a control processor, and a signal processor.

FIELD OF INVENTION

The present invention relates to electron emitting devices, and moreparticularly, to an electron emitting cathode that operates in a radarsystem.

BACKGROUND OF INVENTION

Electron emitting cathodes are used in a variety of devices such ascommunication and radar systems for amplifying radio frequency (RF) ormicrowave electromagnetic signals. For example, an electron emittingcathode may be used within a traveling wave tube (TWT), klystron, orother microwave device. Electrons originating from the electron emittingcathode are focused into a beam propagated through a tunnel or a drifttube generally containing an RF interaction structure. An RF wave ismade to propagate through the interaction structure so that it caninteract with the electron beam that gives up energy to the propagatingRF wave. Thus, the device may be used as an amplifier for increasing thepower of a microwave signal.

The electron emitting cathode may include some form of heater, such asan internal heater disposed below the cathode surface, that raises thetemperature of the cathode surface to a level sufficient for thermionicelectron emission to occur. Alternatively, the cathode may be made toproduce electrons without the aid of a heater, such as for acold-cathode gas tube where the electrons are produced by bombardment ofthe cathode by ions and/or by the action of a localized high electricfield. When the voltage potential of an anode spaced from the cathode ismade positive with respect to the cathode, electrons are drawn from thecathode surface and caused to move toward the anode. A significantenergy level signal is transmitted through this cathode in order toaccelerate the electrons necessary to produce the high power RF output.Each time this occurs more and more electrons are boiled off thecathode. Eventually, the cathode reaches a state where the surface isdepleted. Plasma density within the tube may also increase, resulting inhigh energy electrical discharge (arc) conditions and ultimately failureof the tubes.

When producing RF signals, some users avoid these depletion and highplasma density conditions by operating below the specifications of theequipment. Such a method reduces the capability of the system bycreating RF signals of lower power, yet the method does extend the lifeof the electron emitting cathode and tube and reduces maintenance costs.Other methods continuously operate the tube at full specifications andmay see failures in months rather than years, inducing tens or hundredsof thousands of dollars in maintenance costs for the system.

SUMMARY OF THE INVENTION

The invention consists, generally, of a method for automaticallyreducing power to an electron emitting cathode by sensing an operatingcondition of the electron emitting cathode, calculating a conditionnumber based upon the operating condition, comparing the conditionnumber to a threshold value, and reducing the power to the electronemitting cathode when the condition number is greater than thethreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the invention within a radar system;

FIG. 2 shows a block diagram decision algorithm utilized by theinvention;

FIG. 3 is a flowchart showing process steps utilized in one embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawing figures, FIG. 1 is a block diagram ofcomponents of a radar system 10 incorporating the invention. The radarsystem 10 includes a high voltage power source 12, an RF tube 14, acontrol processor 16, and a signal processor 18. The control processor16 controls the high voltage power source 12 and sends signals to thesignal processor 18 to notify a user of the system 10 of the performanceof the system 10. The RF tube 14 generates an RF signal 15 to send to anantenna 20 based upon the amplitude of the signal from the high voltagepower source 12. A coupler 21 samples the signal from the tube 14 priorto propagation. By controlling the amplitude of the high voltage powersource 12, the amount of energy in the tube 14 is controlled such thatlower amplitudes in the high power voltage source 12 generate lowerenergy levels in the tube 14, and thus lessen the rate of depletion ofthe cathode and the buildup of plasma.

The control processor 16 determines the amplitude of the voltage signalfrom the high power voltage source 12. The control processor 16 receivesdecision parameter signals 22 from the high power voltage source 12and/or the coupler 21 and calculates whether the amplitude of the signalfrom the high power voltage source 12 should be adjusted. If the signalshould be adjusted, then the control processor 16 sends a control signal24 to the high power voltage source 12 to modulate the high powervoltage source 12. The control processor 16 also sends a power levelnotification signal 26 to the signal processor 18 so that the signalprocessor 18 may calculate atmospheric conditions by comparing areceived signal at the antenna 20 to the signal sent from the tube 14.

In a preferred embodiment, the high power voltage source 12 is a DCpower device that provides power to the tube 14 for exciting theelectrons at the cathode and to other elements that help control theelectrons in the tube 14. For example, the high power voltage source 12may provide the power to generate an EM field in the tube 14 forcontaining the electrons, to cool the collector in the tube 14, and/orgenerally operate the tube 14. A measure of the efficiency of the tube,known as the RF power conversion efficiency, is the ratio of the powerof the transmitted RF signal to the power used to excite the electronsat the cathode. This ratio normally ranges from 10% to 60% at fullpower. Another ratio, known as the transmitter system efficiency, is theratio of the transmitted RF signal to the power from the high powervoltage source 12. The transmitter system efficiency, normally, isaround half the RF power conversion efficiency when operating at fullpower. Thus, around half of the power from the high power voltage source12 may be used to power devices within the system 10.

At maximum efficiency, most tubes 14 operate saturated, i.e., completelyon or completely off. There are no intermediate power levels associatedwith the operation of the device. However, operation at saturationmaximizes the rate of depletion of the cathode and the plasma densityinside of the tube 14. Thus, in order to manage the balance betweendepletion, plasma density, and power usage (and particularly power wastefrom drops in efficiency), the control processor 16 applies logic to theoperating parameters of the tube 14 to set the signal of the high powervoltage source 12 at a level that modulates the signal in the tube 14 sothat the operating conditions are managed, as discussed below withreference to FIG. 2.

The tube 14 is an RF power source which converts an electron stream intoan RF frequency by either generating a RF wave or amplifying an RF inputwave. Preferably, the tube 14 is either a klystron, a traveling wavetube, or a magnetron. The tube 14, however, may be any power amplifieror power oscillator. The choice of the type of tube 14 may depend on theapplication, the constraints of the system in which the tube 14 is to beused, cost, and/or availability.

Generally, the tube 14 includes an electron source (i.e., electron gun),an RF interaction portion, and a collector. The high power voltagesource 12 provides a power source to the electron gun which heats thecathode and focuses the emission of electrons from the cathode into anelectron beam. The electron beam travels along the length of the RFsection where it interacts with an RF signal to amplify the signal.After the amplified RF signal is collected from the RF interactionportion of the tube 14, the collector absorbs the remaining electronsdissipating the energy that remains in the tube. The collector may be asingle collector, or may cascade a number of collectors in stages toincrease the efficiency of the collectors by using collectors atintermediate voltages which may then absorb the electrons at voltagesnear optimum.

The most common sources of RF tubes 14 include klystrons, traveling wavetubes (“TWT”), and magnetrons. Which type of RF tube 14 should be usedmay be application specific as these RF tubes 14 have some similaritiesand differences that make them more amenable to certain applications.Each of these types of devices are slow wave devices. The phase velocityof the EM wave in the RF structure is slowed to a velocity approximatelyequivalent to the speed of the electron beam. In this manner, the DCenergy in the electron beam may maximally couple to the RF signal andefficiently transfer energy to the RF signal. However, the energytransfer is not complete, which is why collectors must harness energy atthe end of the tubes.

A klystron creates interaction between the RF signal and the electronbeam at interaction gaps in the wall of the collector anode. At theinteraction gaps, RF cavities are coupled to the anode. An RF signalinput into a first RF cavity is amplified at the first interaction gapand travels to the adjacent interaction gap across the drift space. Inthe final RF cavity along the anode, the amplified RF signal is coupledto a waveguide or other RF transmission line and passed to the antenna20.

Depending when an electron passes the interaction gap, the electron maybe accelerated or decelerated. If an electron passes the interaction gapwhen the RF signal is at a peak value, the electrons are accelerated.Conversely, when an electron passes the RF signal at a minimum value,the electron are decelerated. An accelerated electron may catch a slowedelectron and “bunch” electrons so that the electron beam becomes densitymodulated. As the electron beam continues down the path, the bunchingbecomes greater as additional interaction gaps are crossed. At theoutput RF cavity, the bunching is maximum and the gain in the RF signalis realized. The klystron, then, has a high gain and good efficiency inthat it maximizes transfer from the electron beam. The klystron may alsohave higher average and peak powers relative to other RF tubes. Thebandwidth for a klystron, though, may not be as large as desired, andparticularly may not be as wide as desired at lower power levels.

A TWT has a relatively large bandwidth, but generally has less gain andless efficiency than a klystron. As the power level increases in a TWT,the bandwidth decreases, but still may be a relatively wide bandwidth.The TWT is similar in operation to the klystron, but differs from aklystron in that the interaction between the RF wave and the electronbeam is continuous over the length of the RF interaction portion of thetube while the interaction in the klystron occurs only at theinteraction gaps. In order for the interaction to be continuous, a slowwave structure along the length of the RF interaction portion is used.In cases where the broadest bandwidth is achieved, a helical slow wavestructure is used. Other slow wave structures, such as the ring-barcircuit or coupled-cavity circuits like a cloverleaf, may be used forhigher power applications, but may not have as wide a bandwidth as thehelical slow wave structure. Other power amplifiers are generallysimilar in structure to the klystron and TWT, while power oscillators,such as a magnetron, use different structures to create the RF signalfor transmission.

A magnetron is a compact and efficient power oscillator that may be usedas the tube 14 in the system 10. The magnetron uses a circularconfiguration where the cathode is centrally located. Vanes extendingradially away from the cathode define walls of resonant cavities andattach to the inside wall of the anode. As the electrons are excited atthe cathode and flow outward, interaction between the electrons and theRF wave in the resonant cavities occurs. The RF field extends outwardthrough coupling slots in the wall of a cylindrical anode centeredaround the cathode and enter a coaxial cavity. A coupling slot on theouter wall of the coaxial cavity couples to an output waveguide, whichis the output of the magnetron. While the structure and application ofRF tubes 14 differs, the properties of each structure are known andanalysis of the signal of the high power voltage source 12 may bespecific to the type of RF tube 14 used in the application.

Signal processor 18 receives power level notification signal 26 from thecontrol processor 16. The power level notification signal 26 allows thesignal processor 18 to properly analyze the received signal. The powerlevel notification signal 26 includes the operating power condition ofthe transmitted signal. The signal processor 18 may then adjust thecalibration curve so that proper analysis may occur.

Turning now to FIG. 2, FIG. 2 shows a block diagram of a decisionalgorithm 30. The decision algorithm compares a set of decisionparameters 32 to a set of user defined characteristics. The set ofdecision parameters 32 are passed to a set of parallel comparators 34where the set of decision parameters are compared to the set of userdefined characteristics. A summing block 36 sums the output of thecomparators 34. The summed output is passed to a power level decisionblock 38, which determines the power level.

The set 32, which is denoted by R, is the set of conditions measured bysensors within the high voltage power supply 12 and tube 14. Thesesensors provide information concerning high energy discharge eventswithin the high power tube 14. Depending on the type of RF tube 14,different sensors may be used and may measure different characteristicsof the high power voltage source 12 and the RF tube 14. For example, thesensors may sense the number of arc events that have occurred in thetube 14. Other conditions, such as the average number of arc events in agiven time, and the number of arc events in a specified unit of time.Other conditions such as the average VSWR, change in the average VSWRover time, or amount of change in the VSWR in a given time unit may alsobe conditions passed from the sensors for inclusion in the set 32.

Each of the sensors sends a signal through the decision parameterssignal 22 to the control processor 16 so that the control processor 16may calculate the power level according to the output of the summingblock 36 after the comparators 34 compare the sensor signals 32 to theset of user defined characteristics. The individual elements of the setR may include one sensor signal or may be combinations of sensor signalsaccording to the type of condition the element of the set R is sampling.

The set of user defined sensor threshold condition characteristics,denoted by S, defines a user specified level for each of the values forthe sensor. The set may be defined based on the type of sensor, theoutput range of the sensors, the sensitivity of the sensors toconditions in the system 10, and the probability of failure based uponthe sensor signal. Many of these characteristics may be defined by thetype of RF tube 14 is used and may be set by the type of RF tube 14.Other characteristics may be based on the user's comfort level with thesystem 10 and the ability of the user to predict faults within thesystem 10. Each sensor signal in the set R corresponds to one conditioncharacteristic in the set S, so that the comparators 34 are comparinglike terms.

The comparators 34 receive a single value from the set R and balancethat value against the similar user defined sensor threshold value inthe set S (not shown). If the value from R is greater than the valuefrom S, then the comparator 34 outputs a threshold indicia of 1. If thevalue from R is less than the value from S, then the comparator 34outputs a threshold indicia of 0. Thus, the output of any of thecomparators 34 is a binary value, either a 0 or a 1.

The set of comparators 34 may include n number of comparators 34, wheren is the number of sensors (or defined condition characteristics) in thesystem 10. The n comparators 34 generate a threshold indicia outputvector, M, which includes n components, each component having a value ofeither 0 or 1. The output vector M is passed to the summing block 36. Inan alternative embodiment, one comparator may be used to compare theelements in vector R to the elements in vector S.

Within the summing block 36, each of the values in the output vector Mis summed and divided by the number of sensors, n. The output of thesumming block 36, K, is a scalar measure of the number of conditioncharacteristics that are greater than the defined conditioncharacteristics. The summing block output K varies from a value of zeroto a value of one, and may generally be considered a measure of thepercentage of conditions that exceed the defined conditions. As thepercentage increases, the likelihood of degradation within the tube 14increases.

The scalar K is passed to the power level decision block 38. The powerlevel decision block 38, as will be described further in FIG. 3,determines the power level of high power voltage source 12 which, inturn, sets the stress level on the tube 14. At some predefined thresholdlevel, the output K becomes greater than the threshold, and the systemreduces power to the tube 14. The relationship between the scalar K andthe threshold defines when and how the power in the tube 14 iscontrolled.

In another embodiment of the decision algorithm 30, the output vector ofthe comparators M may be a scaled binary vector. The scaling process mayoccur within the comparators 34 individually, or in the summing block36. If the scaling is done within the comparators 34, then individualentries in the vector M will be scaled prior to passing the outputvector M to the summing block. The n components of the vector M may havevalues other than 0 or 1. The components would have values of either 0or A_(i) where A_(i) is the scaling factor for the i conditioncharacteristic with i varying from 1 to the number of conditions n. Thesumming block 36 then sums the vector M which may have many differentvalues in it. The output K of the summing block may not be limited tovalues less than one, however, because higher scaling factors for someof the conditions could exceed a value of one. It may be beneficial toscale an individual condition with a higher scaling value than one whenan individual condition increases the likelihood of a failure in thetube 14 relative to the likelihood of other individual conditions.Similarly, conditions that are less likely to cause tube fatigue mayhave scaling factors smaller than one to minimize the effects of thatcondition being over-amplified in the overall analysis of the tube 14.

While the scaling factors may be implemented at the comparators 34, thescaling factors may also be implemented in the summing block 36. Whenthe scaling factors are incorporated at the point of the summing block36, then the form of the summing equation is:

$K = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{A_{i}M_{i}}}}$

The scaling factors A may be expressed as a vector having n components.Then, the output of the comparators would be similar to the comparators34 of FIG. 2 in that the result of each comparator would be a value ofzero or one. Whether the scaling occurs at the summing block 36 or thecomparators 34, the scaling factors may be set by either the user orwithin the system 10 according to the type of components and sensors inthe system 10. For example, a user may wish to change the scaling factorof a sensor that the user believes has higher predictive capabilitiesfor degradation and, ultimately, failure of the tube 14.

In another embodiment of the algorithm, the output of the comparators 34may vary from zero to one. In such an embodiment, an additional step ofcapturing the magnitude of the difference between the sensed conditionand the defined condition characteristic would vary the output of thecomparator between zero in one. In the simplest calculation, the outputwould vary linearly such that the output for a comparator 34 may equalthe difference between the sensed condition and the defined conditioncharacteristic divided by the defined condition characteristic plus anoffset of 1. As the sensed value approaches the defined conditioncharacteristic, the value from the comparator approaches 1. As thesensed value exceeds the defined condition characteristic, the outputvalue exceeds one. In this manner, when a sensor senses anextraordinarily high value, the output of the comparator 34 varies theresult high. Similarly, low values in the sensor vary the result low,preferably with a lower limit of zero. It may also be desirable to setan upper limit for any one comparator output to limit the possibilitythat a single sensor alone could trigger a power change by overwhelmingthe value of K over the value of the threshold. In other methods ofvarying the output of the comparators 34, the output may be variedparabolically, along a bell curve, or based on any other function thatvaries the output according to a need of a user or the system 10.

Turning now to FIG. 3, FIG. 3 is a flowchart of steps in a component ofFIG. 2 according to an embodiment of the invention. In step 50, sensordata and system parameters are received. A decision step 52 determineswhether the system is in override. If the system is not in override,then the decision parameter set R 32 is calculated in step 54 andcompared to the user defined conditions S in step 56. The summing blockoutput K is calculated in step 58. A decision block 60 determines if thesystem is in reduced power mode. If the system is not in the reducedpower mode, then decision block 62 compares the summing block output Kto the threshold T. If the value of the result of the summing block K isgreater than the threshold T, then power is reduced in step 64. If thevalue of the result of the summing block K is less than or equal to thethreshold T, then the system remains at full power in step 66. If, instep 60, the system is already in reduced power, then decision block 68compares the summing block output K to the threshold T. If the value ofthe result of the summing block K is greater than or equal to thethreshold T, then the system remains at reduced power in step 70. If thevalue of the result of the summing block K is less than the threshold T,then power increases in step 72.

In step 52, if the system is in override, then the method maintains fullpower in step 66. Override may be necessary at times when the radarsystem must maintain full capability regardless of sensor output fromthe tube 14. Such a circumstance may exist when inclement weather ispresent, or expected. Thus, a user may specify the override condition tokeep the steps used for determining poor operating conditions within thetube 14, and maintain full power regardless of operation. Override,however, makes the system more susceptible to a failure caused by thetube 14 failing.

Steps 54-58 of the method of FIG. 3 perform the calculations in thedecision algorithm 30. The decision parameter set R is calculated fromthe sensor feedback received from the system 10. The sampling rates fordifferent sensors and storage of information from the sensors may residewithin the control processor 16. It may be beneficial to use rollingaverages of sensor data such that data from the sensors may be sampledand kept in memory within the control processor 16. In this manner,multiple samples from a sensor may be averaged forming a roughapproximation of a low pass filter. Other sensors may use single samplepoints for extraction of entries in the decision parameter set R. Themethod of FIG. 3 may repeat at any rate either defined by a user, at adefault rate, or triggered by events within the system. For example, therate at which the method repeats may be a function of the sampling speedof the sensors. It may be beneficial to limit the rate at which themethod processes the data parameter set between the slowest rate atwhich a sensor is sampled and the fastest rate at which a sensor issampled. If the method repeats at the fastest rate, then each time thefastest sensor refreshes its value, the method will process the decisionparameter set R. If the method repeats at the slowest sensor rate, theneach sensor will have sampled a new sample between calculations of thedecision parameter set R.

Steps 60-72 of the method of FIG. 3 set the power level for the system10. In the preferred embodiment of FIG. 3, the power level is set to oneof two states. The system 10 performs at full power or reduced power. Inthe preferred embodiment, the reduced power is approximately 50% ofstandard maximum output. However, other power levels may suffice, andmay be set by the user according to the system 10 of the user. Also, auser may set multiple states to drop the power level incrementally. Forexample, if the condition number K remains high a certain time after thepower has been reduced, then the method may further reduce the powerlevel. The further reduction may be as a percentage of the reduced powerlevel, or as an absolute drop of power. In this manner, a user maydefine a stairstep method of reducing power to the system 10 to maintainthe desired operation of the system 10.

In another embodiment, the threshold may change according to the currentoperating condition (i.e., power level) of the system 10. For example,if the system is in reduced power, then the threshold may be set lowerthan if the system was in full power. Then, the next time the method ofFIG. 3 processed the data set and compared the condition number K to thethreshold, the power level would be returned to full power only if thecondition number was less than the lower threshold. By implementing thelower threshold when in reduced power mode, a hysteresis is establishedthat gives the system 10 more time to operate at the lower power levelwhich may help to prolong the life of the tube by providing greaterrecovery time within the tube at lower power levels. Once the conditionnumber K drops below the lowered threshold and the system 10 returns tofull power, then the threshold may also increase to its full value sothat the system may run at full power until the conditions sensed by thesensors exceed the threshold number. In this embodiment, then, the powerlevel does not rise and fall regularly as the condition number K variesincrementally around the threshold number. This same method ofintroducing hysteresis in the power level may also be implemented in themulti-state power level described above.

In another embodiment of the method of FIG. 3, the power level may alsobe set according to the difference between the condition number K andthe threshold T. For example, the method may reduce power linearly withrespect to the difference between the condition number K and thethreshold T. In such a method, as the condition number K increases insize, the amount of power reduction increases. As the condition numberreturns to a normal operating condition, then the power level returns tomaximum power. Mathematically, the power level may be represented as:

P _(cur) =P _(max) −C(K−T)

where P_(cur) is the current operating power level, P_(max) is themaximum power and C is a scaling factor representing the amount of powerreduction for an incremental change in the condition number. In a linearsystem, C is a constant. Other scaling factors may be used to vary thechange in power level other than linear. For example, the differencebetween the condition number and the threshold may be squared, or raisedto some other power, in order to vary the power exponentially withrespect to the difference between the condition number and thethreshold.

The methods that adjust the power level may include other logicalalgorithms such as a fuzzy logic controller, a neural networkcontroller, combinations of these controllers, or other I/O algorithms.In implementing the different controller algorithms, criteria such asstability, efficiency and noise should be considered.

While the invention has been shown in embodiments described herein, itwill be obvious to those skilled in the art that the invention is not solimited but may be modified with various changes that are still withinthe spirit of the invention.

1. A method of reducing power to an electron emitting cathode,comprising the steps of: a. sensing an operating condition of saidelectron emitting cathode; b. calculating a condition number based uponsaid operating condition; c. comparing said condition number to athreshold value; and d. reducing power to said electron emitting cathodewhen said condition number differs from said threshold value by apredetermined amount.
 2. The method of claim 1, wherein said sensingstep further comprises sampling a plurality of sensors.
 3. The method ofclaim 2, wherein said calculating step further comprises balancing saidsamples of said plurality of sensors against a plurality of sensorthreshold values such that the number of said samples is equal to thenumber of said plurality of sensor threshold values.
 4. The method ofclaim 3, further comprising balancing each of said samples of saidplurality of sensors against a corresponding one of said plurality ofsaid sensor threshold values.
 5. The method of claim 4, wherein saidbalancing step further comprises incrementing said condition number by aunit value when the value of one of said samples of said plurality ofsensors is greater than a corresponding value of one of said pluralityof sensor threshold values.
 6. The method of claim 4, wherein saidbalancing step further comprises incrementing said condition number byan amount proportional to the difference between the value of one ofsaid samples of said plurality of sensors and a corresponding value ofone of said plurality of sensor threshold values.
 7. The method of claim1 wherein said reducing step further comprises reducing the power tosaid electron emitting device to fifty percent of the maximum power ofsaid electron emitting device.
 8. The method of claim 7, furthercomprising the step of lowering said threshold after said reducing step.9. The method of claim 8, further comprising the step of increasing thepower to said electron emitting cathode when said condition number fallsbelow said lowered threshold.
 10. The method of claim 9, furthercomprising the step of returning said threshold to the original value ofsaid threshold when said power is increased.
 11. The method of claim 7,further comprising the step of further reducing the power to saidelectron emitting cathode when said condition number remains greaterthan said threshold.
 12. The method of claim 1 wherein said reducingstep further comprises reducing power by an amount proportional to thedifference between said threshold and said condition number.
 13. Asystem for controlling the power to an electron emitting cathode;comprising: a. a plurality of sensors configured to sense operatingconditions of said electron emitting cathode; b. comparators configuredto compare said operating conditions to a plurality of sensor thresholdvalues and to output a threshold indicia; c. a summing block configuredto sum said threshold indicia, said sum of said threshold indicia beinga condition number; and d. a power level decision block configured toreceive said condition number and to compare said condition number to athreshold value to determine a power level for said electron emittingcathode.
 14. The system of claim 13, wherein said comparators arefurther configured to increment said condition number by a unit valuewhen the value of one of said operating conditions from said pluralityof sensors is greater than a corresponding value of one of saidplurality of sensor threshold values.
 15. The system of claim 13 whereinsaid comparators are further configured to increment said conditionnumber by an amount proportional to the difference between the value ofone of said operating conditions of said plurality of sensors and acorresponding value of one of said plurality of sensor threshold values.16. The system of claim 13, wherein said power level decision block isconfigured to reduce the power to said electron emitting device to fiftypercent of the maximum power of said electron emitting device when saidcondition number is greater than said threshold.
 17. The system of claim16, wherein said power level decision block is further configured tolower said threshold when the power to said electron emitting device hasbeen reduced.
 18. The system of claim 17, wherein said power leveldecision block is further configured to increase the power to saidelectron emitting cathode when said condition number falls below saidlowered threshold.
 19. The system of claim 18, wherein said power leveldecision block is further configured to return said threshold to theoriginal value of said threshold when said power is increased.
 20. Thesystem of claim 16, wherein said power level decision block is furtherconfigured to further reduce the power to said electron emitting cathodewhen said condition number remains greater than said threshold.
 21. Thesystem of claim 13 wherein said power level decision block is configuredto set the power to said electron emitting cathode by an amountproportional to the difference between said threshold and said conditionnumber.
 22. A data structure for controlling the power to an electronemitting cathode, comprising: a. a set of operating conditions; b. a setof operating condition thresholds; c. a comparator configured to comparesaid set of operating conditions to said operating condition thresholds,said comparator further configured to output a threshold indicia vector;d. a summing block configured to sum said threshold indicia vector, saidsum being a condition number; and e. a power level decision blockconfigured to receive said condition number and to compare saidcondition number to a threshold to determine a power level for saidelectron emitting cathode.
 23. The structure of claim 22, wherein saidcomparator is further configured to increment said condition number by aunit value when a value of one element of said set of operatingconditions is greater than a corresponding value of one element of saidset of operating condition thresholds.
 24. The structure of claim 22wherein said comparators are further configured to increment saidcondition number by an amount proportional to the difference between avalue of one element of said set of operating conditions and acorresponding value of one element of said set of operating conditionthresholds.
 25. The structure of claim 22, wherein said power leveldecision block is configured to reduce the power to said electronemitting device to fifty percent of the maximum power of said electronemitting device when said condition number is greater than saidthreshold.
 26. The structure of claim 25, wherein said power leveldecision block is further configured to lower said threshold when thepower to said electron emitting device has been reduced.
 27. Thestructure of claim 26, wherein said power level decision block isfurther configured to increase the power to said electron emittingcathode when said condition number falls below said lowered threshold.28. The structure of claim 27, wherein said power level decision blockis further configured to return said threshold to the original value ofsaid threshold when said power is increased.
 29. The structure of claim25, wherein said power level decision block is further configured tofurther reduce the power to said electron emitting cathode when saidcondition number remains greater than said threshold.
 30. The structureof claim 22 wherein said power level decision block is configured to setthe power to said electron emitting cathode by an amount proportional tothe difference between said threshold and said condition number.